Steel rule dies are used in cutting many of the fabricated items we take for granted. The materials cut by dies range from soft metals and tough plastics through foams, leathers, and cloth, to frail films, papers and even food items. The end products of this type of die cutting include thick foam cushions, shoe parts, decals, metal foil trays, and bakeware.
Essentially any product that can be produced in sheet form may be cut with steel rule dies. Other similar die types, such as clickers, can be made with the same methods as steel rule dies, although the die material may be slightly different.
These dies have evolved from the blacksmith's dies of heavy wide strip which were sharpened on one edge and hammered into leather goods. Today, however, steel rule dies have become a specialized industry. The shapes of these dies are often relatively sophisticated, and made from thin rule stock, which is specially treated steel strip that is flexible, yet holds its sharpened edge. The dies are formed and then placed into holding matrices such as slotted plywood or cast polymers. Steel plates may be added to distribute press forces onto the holding matrices.
The steel rule industry is presently estimated to produce revenues of between four hundred million and six hundred million dollars per year. The annual production of about one hundred twenty-two million feet of rule, at an average cost of between thirty-four cents and thirty-eight cents per foot, represents about forty-four million dollars of this total. It is further estimated that at least thirty-five percent of the total footage of steel rule produced is turned into scrap, as a result of the methods currently used to manufacture the dies.
The formed steel rule dies are usually either pressed into laser cut grooves or into intricately sawn slots in a plywood base, and must be formed to fit the slots. The alignment of parts by mere use of the slots as guides is rarely successful if the shape of the rule is not pre-formed to conform to the shape of the slots. In addition, a frequently used technique provides for frequent lands in the plywood parts that hold the centers of cutouts within the die frames. The lands are accommodated by cutting matching notches into the steel strip so that the strip goes over the lands without making contact.
The dies are made from any of a series of specialty steels. The stock is formed into a coiled strip and then sharpened on one edge of the strip. The coils are difficult to handle, so the strips are usually supplied in a flat cutoff form. The die is constructed from one or more sections of this strip and may be bent at various sharp angles, or into gradual curves, or used as straight sections, depending on the desired shape. While the use of steel rule dies is not the only way to make repetitive and essentially identical cut shapes, such use constitutes one of the lowest cost and most popular methods.
The steel rule dies are made by bending strips of the edge-sharpened steel into the required intricate shapes. The formed strip is then attached to a backing that permits pressure to be applied by a press or powered stamping machine to cut out the materials. The bending is sometimes done by hand, or, at other times by machine. Regardless of the method of manufacture, making the bends requires the skill of experienced artisans because the steel strip, as it is bent, tends to spring back, due to the elastic properties of steel.
Within the range of elastic response, prior to the yield point where a deformation becomes permanent, there is considerable spring back whenever a bend is made. The experienced bender must estimate the amount of spring back or spring back (terms used interchangeably herein) and overbend the strip by an appropriate amount to accommodate for the spring back.
In the operator-based system now in use, the operator must anticipate the memory of the steel strips and overbend each change in direction so that when the steel memory causes a spring back to occur, the spring back is just the right amount to make the desired final shape. This overbending by "feel" is rarely perfect. Minor problems can often be cured by forcing the strip into the cut grooves in the plywood backing support, but the cumulative effect of errors on complex shapes can deform the die so that it no longer fits the grooves. As a result, there may be a loss of materials and labor.
Thus, the manual approach to making steel rule dies requires well trained and experienced operators, who usually command relatively high pay. Furthermore, the time required for manual production of steel rule dies is exorbitant.
Automation of this process by automatic bending machines has been an especially difficult problem for designers. This difficulty is due to the high variability of steel rule resulting in a wide range of spring back memory from lot to lot. Such a bending machine would have to be calibrated for every lot of steel strip used, and a new spring back algorithm created for each and every lot. Further complicating the process is the fact that the properties of rule may vary within a single lot of steel strip manufactured.
Thus, there is a need for a steel rule die manufacturing method that is relatively independent of the spring back variations, and which can control these variations so that bends remain highly accurate. Until now, such a method has not been available.
There is an additional complication in die manufacture. Even if the strip properties are compensated for by a machine, the properties for an initial bend are different than the bending properties of a strip bend corrected by a second bend at the same place. This secondary bend or correction of bend also must be compensated for.
There is also a need to reduce waste in the rule bending process. When the strip or rule is delivered in short, easy-to-handle lengths there is a high amount of waste, since the initial portion and tail ends of each strip are often discarded. Up to 40% of each precut strip is wasted in bending, since the strips are only partially used, or the die manufacture requires a front grip area and a rear grip area that becomes waste. As noted above, the memory or spring back also creates waste. There is a need for a bending process that can exactly replicate a form or at least create a duplicate where the incremental steps in bending approach the exact duplication of the original form.
At present the problems noted above are major ones that prevent effective and exact steel rule die manufacture and further prevent the automation of this manufacture, although there have been several attempts to overcome these problems.
One such attempt is the BBS-101 & caddie made by Tsukatani Blade Manufacturing Co., Ltd. of Osaka, Japan. In this machine a massive memory of blade material properties is stored in the machine's computer memory. Each machine is individually calibrated against these data files to provide a predictable calculation of the amount of bend required. The scale of the required bending and the localized and individual lot properties of the steel are not accounted for, however. As a result, the bend accuracy is not good since there are springback fluctuations in the blade strip materials that are not taken into account. Other problems include stretching or compression of the strip in bends, jaw slippage, and the fact that the radius of the bends is restricted by a head design that has fingers which slide on the perimeter of a round grip bar. This feature precludes bends being spaced closely together, bends smaller than the round grip bar, or larger than a factor of 1.3 times the grip bar radius.
As a result, this machine can make only 50 to 60% of the simpler needed bends.
Another approach is describe in Archer et al., U.S. Pat. No. 4,627,255, Dec. 9, 1986. This patent describes a system in which the steel rule is passed through a bending head, and an electrical contact used to sense when the rule is no longer in contact with the bending head, and thus provides a rough measure of the resulting shape of the die so far. However, this device does not provide a precise method for measuring the die shape produced at this stage of the process. This invention furthermore does not withdraw the bending head at the point of cutoff, thus requiring trimming of the finished die, with resulting excessive scrap.
Other prior art includes a patent by Graboyes, describing a method for registering steel rule dies where registration holes are used, not in the forming, but for registration within a cutter (U.S. Pat. No. 4,341,008). Registration within a cutter is also noted in U.S. Pat. No. 4,397,095, but not the registration and alignment of bends in making dies.
Another Patent, for a Method of Making Steel Rule Type Piercing and Blanking Dies, issued to Whitecotton et al. (U.S. Pat. No. 4,226,143) is representative of the present state of the art. This method uses a punch die blank that is covered with a tape layer that allows a clearance distance. The steel rule elements are assembled against the punch die blank, so that the shape of the punch blank is accurately maintained. Repetitive dies can be accurately made with this method, but the method does not address the bending or creation of complex steel rule dies.
The prior art also contains a number of wire benders. These devices are designed to handle either wire ends in electrical/electronic assembly operations (which have little in common with the diverse requirements of a strip bending) or they are designed to bend wire shapes. They are generally not applicable to the current application.